The invention relates to thermoreactive hot-melt adhesives and, in particular, to polyurethane-based thermoreactive hot melt adhesives.
Polyurethane adhesives (PUR-hot-melt adhesives) of prepolymers based on polyadipic acid derivatives or polyglycol derivatives and polyisocyanates are known, for example from EP-B-455 400. These PUR-hot-melt adhesives are used for the bonding of synthetic materials, glass, metal, leather or wood and cure irreversibly within a few days, due to the effect of the humidity in the air or in the materials being adhered. The PUR-hot-melt adhesives provide a bond that has good heat stability and resistance against chemicals. The solidification times (i.e., setting times) of the PUR-hot-melt adhesives may be adjusted to a range from seconds to minutes, and low melting viscosities and a fast solidification, and a good cold-elasticity can be achieved after the application. See, for example, DE-A-38 27 224, DE-A-41 14 220, EP-A-0 354 527.
A drawback of these products is the storage capability, as traces of water during the preparation or the storage of the adhesives may already lead to a rise in viscosity and to a premature curing and thus to failure times or faulty adhesion due to obstructed lines and nozzles of the application devices. An additional drawback is the gas evolution of carbon dioxide during the reaction with water to polyurea, whereby the adhesive could foam and alter the desired position of the joining parts.
Polyurethane adhesives that can be thermally activated, based on prepolymers and blocked isocyanate groups, are also known. When the temperature rises, first the isocyanate groups are set free and then, by an additional effect of humidity on the reactive isocyanate groups, the cross-linking reaction takes place and thus the adhesion process. The parts to be adhered have to present a sufficient humidity concentration, such that adhesions of impervious materials are hardly possible.
EP-A 0 431 413 describes a thermally curable PUR-powder based on polyurethane with encapsulated isocyanates, whereby the de-activation of the isocyanate groups is not brought about by capping, but by a blocking layer on the surface of the particle. Particulate materials are used as components in the formulation. In order to generate powder-shaped products, it is necessary to add the reaction mixture as a melt to an inert solvent/emulsifier system.
In its broadest sense, the invention provides a thermally curable hot-melt adhesive composition, comprising
(A) a prepolymer having isocyanate groups, a number average molecular mass Mn of 700 to 6000 and wherein 50 to 100% of the reactive isocyanate groups of the prepolymer are blocked, said prepolymer being the reaction product of
(1) at least one straight-chain polyester which is at least semi-crystalline with
(2) at least one straight-chain polyester selected from the group consisting of amorphous polyesters and liquid polyesters,
(3) optionally at least one polyether, and
(4) at least one diisocyanate; and
(B) at least one reactive component selected from the group consisting of diamines, epoxide adducts of diamines, and polyalcohols.
The hot-melt adhesive composition of this invention does not have the above-mentioned drawbacks, but do have a high resistance against humidity and a high storage capability, in particular when the joining parts are precoated with the adhesive and if the actual pasting is carried out later.